JpGU-AGU Joint Meeting 2017

Presentation information

[JJ] Poster

S (Solid Earth Sciences) » S-MP Mineralogy & Petrology

[S-MP44] [JJ] Physics and Chemistry of Minerals

Sun. May 21, 2017 5:15 PM - 6:30 PM Poster Hall (International Exhibition Hall HALL7)

[SMP44-P13] Grs50Prp50 garnet-bearing composite inclusion in Cr-rich pyrope from Garnet Ridge, the Colorado Plateau

*Hiroshi Suzuki1,2, Tomohiro Takebayashi2, Takeaki Saito1, Kunihiko Sakamaki2, Yoshihide Ogasawara1,2 (1.Department of Earth Sciences, Waseda University, 2.Department of Earth Sciences, Resources and Environmental Engineering, Graduate school of Creative Science and Engineering, Waseda University)

Keywords:Grossular-pyrope garnet, Garnet Ridge, Kimberlite, Laser Raman spectroscopy, Colorado Plateau, Cr-rich pyrope

Garnet having near Grs50Prp50 composition is very rare in nature because of the large difference in ionic radii between Ca2+ and Mg2+. So far, only two occurrences have been reported from Garnet ridge, Arizona (Wang et al., 2000) and the Kokchetav UHP Massif, Kazakhstan (e.g., Ogasawara et al., 2000; Sobolev et al., 2001). At Garnet Ridge, Wang et al. (2000) described four grains of Grs50Prp50 garnet as a constituent of composite inclusions in pyrope-rich garnet in kimberlitic diatremes. In the Kokchetav UHP Massif, Grs50Prp50 garnet is a major constituent mineral of UHP dolomite marble, and contains abundant microdiamonds. Takebayashi et al. (2017) has stated that CaO8 and MgO8 clustering around a SiO4 tetrahedron stabilized ca. Grs50Prp50 compositions on the basis of the overlapping of R(SiO4)4- Raman bands corresponding to Grs (372 cm-1) and Prp (364 cm-1), and considered that two main factors controlled the formation of this strange garnet; (1) the bulk chemistry of the host rock (Ca:Mg = 1:1) and (2) UHP conditions.
Recently, we discovered one grain of Grs50Prp50 garnet from the Garnet Ridge; the garnet occurs as a constituent of composite inclusion in the host Cr-rich pyrope (Group A by Sakamaki et al., 2016), which is of garnet lherzolite origin. Cr-rich pyrope (Group A) is an original material for Cr-poor pyrope (Group B) during mantle metasomatism. The found composite inclusion, which shows spherical form measuring 150 mm across, consists of pargasite and dolomite with minor Cr-spinel, phlogopite and apatite. The other composite inclusions consist of pargasite, dolomite, Cr-spinel with minor apatite and magnesite. We conducted laser Raman spectrometry on this Grs50Prp50 garnet, and focused on the band attributed to R(SiO4)4- at 365 cm-1. The overlapping of R(SiO4)4- bands corresponding to Grs and Prp in a single Grs50Prp50 crystal was observed. Our results of Raman spectrometry were consistent with those of the Kokchetav Grs50Prp50 garnet by Takebayashi et al. (2017).
Almost all composite inclusions contain dolomite/magnesite and show rounded or spherical form. This suggests that these composite inclusions was trapped carbonate-silicate melt during the mantle metasomatism. The Grs50Prp50 garnet in the found composite inclusion was formed from such trapped melt which had the bulk chemistry, near Ca:Mg = 1:1, at very high pressure.
The Grs50Prp50 garnet described by Wang et al. (2000) could have formed by the same process from trapped carbonate-silicate melt, and the inclusion Grs50Prp50 garnet was not in equilibrium with the host pyrope-rich garnet. Their interpretation about the genesis of Grs50Prp50 garnet including very low formation temperature based on the coexistence with the host may be wrong.
.
References
Ogasawara, Y., Ohta, M., Fukasawa, K., Katayama, I., Maruyama, S., 2000, Island Arc, 9, 400-416.
Ogasawara, Y., Sakamaki, K., Takebayashi, T., Suzuki, H., Saito, T., 2016, AGU Fall Meeting, SR33A-2673.
Sakamaki, K., Sato, Y., Ogasawara, Y., 2016, Progress in Earth and Planetary Science, 3, 1-17.
Sobolev, N.V., Schertl, H.-P., Burchard, M., Shatsky, V.S., 2001, Doklady Earth Science, 380, 791-794.
Takebayashi, T., Saito, T., Suzuki, H., Sakamaki, K., Ogasawara, Y., 2017. JpGU Meeting Abstract.
Wang, L., Essene, E.J., Zhang, Y., 2000, American Mineralogist, 85, 41-46.